The present invention relates to a memory element and a memory device capable of storing binary or more information by a change in an electrical characteristic of a memory layer including an ion source layer.
DRAM (Dynamic Random Access Memory), which is fast in operation and high in density, has been widely used as RAM (Random Access Memory: random access memory) in information devices such as a computer. However, since a manufacturing process of the DRAM is complex as compared with an ordinary logic circuitry LSI (Large Scale Integration) used in electronic devices or with a signal process, a production cost is high. Also, the DRAM is a volatile memory in which information is lost when power is turned off. Thus, it is necessary to frequently perform refresh operation, that is, to perform operation of reading out the written information (data), performing re-amplification, and performing rewrite again.
Accordingly, a flash memory, FeRAM (Ferroelectric Random Access Memory) (ferroelectric memory), MRAM (Magnetoresistive Random Access Memory) (magnetic memory element) and so forth for example have been proposed as a nonvolatile memory in which the information is not lost even when the power is turned off. In this memories, the information written therein can be retained continuously for a long period of time even when the power is not supplied.
However, each of the various nonvolatile memories mentioned above has both advantages and disadvantages. The flash memory is high in integration, but is disadvantageous in terms of operation speed. The FeRAM has a limit in microfabrication for achieving high integration, and also has a problem in terms of manufacturing process. The MRAM has a problem of power consumption.
Accordingly, a new type of memory element, which is advantageous particularly for the limit in the microfabrication of a memory element, has been proposed. This memory element has a structure in which an ionic conductor containing a certain metal is sandwiched between two electrodes. In this memory element, either one of the two electrodes includes the metal contained in the ionic conductor. Thus, when a voltage is applied between the two electrodes, the metal included in the electrode diffuses as an ion into the ionic conductor. Thereby, an electrical characteristic such as a resistance value or capacitance of the ionic conductor is changed. For example, Patent Document 1 and Non Patent Document 1 each describes a configuration of a memory device utilizing this property. In particular, Patent Document 1 proposes a configuration in which an ionic conductor consists of a solid solution of chalcogenide and a metal. Specifically, it consists of a material in which Ag, Cu, and Zn form a solid solution in AsS, GeS, and GeSe, and either one electrode of two electrodes contains Ag, Cu, and Zn.
Patent Document 1: Japanese unexamined PCT publication No. 2002-536840
Non Patent Document 1: NIKEEI ELECTRONICS, the issue of Jan. 20, 2003 (page 104)